If the
Universe is now expanding, then it
must
have been smaller in the
past.
The further backinto the past you go, the
smaller the Universe was.

If you
go back far enough in time
the entireUniverse seems to have existed at one tinypoint.
This point was the origin (beginning) of
the Universe.
The big bang was an explosion at this point and theUniverse has been expandingaway from
this pointever
since.

If you look
at how fast the Universe is expandingtoday
then you can make an estimate
(guess)
of the length of time the Universe has been expanding
in order to occupy the size it has
today.
This length of time is the
age of the Universe.

It is
noteasy
to estimate the age of the Universe.Gravity is an
attraction between all the matter in the
Universe.
This attraction tends to slow the rate of
expansiondown.
There is another force (called dark energy) which
tends
to
push matter apart and speed the rate of
expansion up.

It is believed that the Universe is expandingmore quickly today than it has done in the past.
Taking this into account,
scientists estimatethe age of the Universe
to be about
14 billion years
(1 billion = 1,000,000,000 or 1 thousand
million).